The present invention generally relates to an optical information recording medium, and especially an optical disk from which high density recorded information may be reproduced using laser beams or the like.
Generally, a technique is known of recording minute signal pits on a substrate having optical spot diameters which are on the order of a light wavelength used upon reproduction. Such a technique is put into practical use in the form of a read only type (ROM=Read Only Memory) optical disk such as compact disk (CD) and a laser video disk (LVD). Another known technique is directed to an optical phase change type optical disk which uses chalcogenide as a recording film material and has been put into practical use as a write once type (WORM=Write Once Read Many) optical disk capable of recording and reproducing signals or a rewritable optical disk capable of recording, reproducing and erasing of signals.
In a read only type optical disk, a signal track formed of small concave and convex pits is provided in advance between a transparent disk substrate such as resin or the like and a reflective film such as Al or the like. During signal reproduction, a scanning operation is effected on a signal track having optical spots by focusing laser beams. The laser beams are reflected at locations where pits do not exist and are substantially diffracted at locations where pits exist. By using an optical detector, the location of the signal pits can be discriminated in accordance with a magnitude of the reflected light.
In an erasable optical phase change type of optical disk, signal marks are formed in accordance with a crystal condition of a recording film material. The recording film, when melted and quenched upon application of laser beams, becomes a crystalline state. Optical constants (refractive indexes, extinction coefficients) of recording marks located on the recording film differ according to a non-crystalline state or a crystalline state of each location, and thus a reflected quantity of light recording mark locations also varies accordingly. Signals are detected in accordance with the magnitude of reflected light. In order to erase the signals, the recording film is increased in temperatures by application laser beams, having a lower power than those used during recording, so as to return recording mark location to the non-crystalline state.
As a recording film material, typically a material is used as above which changes in optical phase between a non-crystalloid state and a crystal state and which has Te, In, Sb, Se or the like as principal components, or a material is used which reversibly changes in optical phase between two different crystal states. One merit of optical change recording is that information signals can be overwritten using a single laser beam. When the laser output is modulated in accordance with the information signal between two levels, i.e., a recording level and erasing level, and is applied to a recorded information track, a new signal can be recorded while the existing information signal is erased (Japanese Patent Laid-Open Publication No. 56-145530).
The write once type optical phase change optical disk also records and reproduces signals with the use of the reflection index of a recording film.
It has been proposed to provide a disk construction in which a structural phase change is caused between a non-recorded portion and a recorded portion with respect to the wavelength .lambda. of reproduction laser beams (Japanese Patent Laid-Open Publication No. 3-41638, and Japanese Patent Laid-Open Publication No. 3-157830). When the recorded portion (recording mark) is read out reflection index change, a suitable reproduction signal cannot be obtained without the area of the recording condition of the range sufficiently larger as compared with the size of the laser beams to be used for the reproduction. This is because the magnitude of a reflected light beam is proportional to a value averaged with the respective area and the optical intensity distribution being weighted upon the reflection index of the recording mark and the reflection index of the peripheral unrecording region as the optical intensity of the reproduction beams is generally Gausian distribution and is spread more externally than the optically phase changed recording mark. In the case of the structural phase change construction, the structural phases of the recorded portion and the non-recorded portion are different, and the reflected quantity of light changes in accordance with the extent to which the structural phases interfere with each other. Preferably, the change in the reflected quantity of light is largest when the structural phase difference of the reflective beam in the recorded portion and the non-recorded portion is (1+2n) .pi. radians (n is an integer). The effect of interference is largest and therefore, the change in reflected light intensity is larger when the reflection intensity from the recorded portion is equal to the reflection intensity from the peripheral non-recorded portion. A larger reproduction signal can be provided when the recording marks are smaller than the size of the reproduction beam. Thus, a larger signal magnitude is provided, and thus a higher density recording operation can be effected using a recording mark having an area which is smaller than that of the reflection index change reproduction construction with respect to a reproduction operation using the same reproduction optical beam.
In the conventional optical disk, the limit of recording density is determined by the extent to which signal marks can be made small and still be detected using light spots focused up to the wavelength limit. The size of the light spots is determined by the wavelength of the laser beam and the numerical aperture (NA) of the objective lens for focusing the laser beam. The light spot can be made smaller, as the wavelength of the laser beam is made shorter or the numerical aperture of the objective lens becomes larger. The improvement in the recording density can be effected only with the development of a shorter wavelength laser and/or a larger numerical aperture objective lengths, which cannot be realized easily.